The turbine rotor assembly of the steam turbine is configured by, for example, inserting moving blades one by one along a circumferential direction from a notch groove formed in a root portion of a rotor disk formed along a circumferential direction of a turbine rotor, and lastly fixing a tightening part such as a notch blade.
The tightening part is being devised in various ways from various viewpoints such as mechanical strength, turbine efficiency, and weight balance. For example, since the tightening part is fixed to the notch groove formed in the root portion of the rotor disk, it does not have a root portion. Therefore, a load is applied to the moving blades on both sides of the tightening part to maintain the assembled state against, for example, a centrifugal force applied to the tightening part. Accordingly, it is preferable that the tightening part's weight is reduced as low as possible in order to reduce the load applied to the both-side moving blades as small as possible.
As the tightening part, there are used, for example, a stopper of which weight is maximally reduced, a stopper block having a structure of the root portion only with an effective blade part and the like removed, a notch blade having the same blade portion as other moving blades, and the like. And, an appropriate one is selected to use from the above tightening parts depending on the strength design and the like of turbine stages.
The above tightening parts have a weight different from the moving blades which mainly configure a turbine moving blade cascade and are formed based on theoretical calculation, so that the more the weight is reduced, the more the weight balance is lost as the turbine moving blade cascade. Therefore, it is also necessary to have moving blades for weight adjustment, so that the tightening part does not become a vibration generating source of the turbine rotor.
Meanwhile, further improvement of performance of the steam turbine is demanded for prevention of global warming. For example, to prevent a stage loss from increasing, there is a tendency to adopt the notch blade as the tightening part without adopting the stopper block not having a steam passage portion. And, it is also tried to use titanium or the like to produce the notch blade. One of the advantages to use titanium as a material for the notch blade is light weight that the weight is about 60% of iron and steel type material. But, the titanium also has disadvantages that its processability is bad and it is expensive.
The structure of a conventional turbine moving blade cascade is described below.
First, a conventional turbine moving blade cascade having a stopper block as a tightening part is described.
FIG. 22 is a schematic view of a conventional turbine moving blade cascade 400 having a stopper block 410 as a tightening part as viewed from the upstream side in a turbine rotor axial direction. FIG. 23 is a plan view of the stopper block 410 as viewed from the circumferential direction. FIG. 24 is a partial magnified view of the turbine moving blade cascade 400 having the stopper block 410. FIG. 25 is an exploded perspective view showing a mounting state of the stopper block 410. FIG. 26 is a plan view of a moving blade provided with a groove 415 for adjustment of a weight balance as viewed from the circumferential direction. FIG. 22 shows numbers corresponding to the quantity of implanted moving blades 411.
The turbine moving blade cascade 400 shown in FIG. 22 has 147 moving blades 411 disposed in the circumferential direction excepting the stopper block 410. As shown in FIG. 23, the stopper block 410 has a structure with only a root portion from which an effective blade part and the like are removed and is fixed between the moving blades 411 as shown in FIG. 24.
As shown in FIG. 25, plural root grooves 421 are circumferentially formed on both side surfaces of the outer circumferential portion of a rotor disk 420, and hook portions 411b formed on a root portion 411a of the moving blade 411 are fitted into the root grooves 421 of the rotor disk 420. The moving blade 411 is inserted via a cutout portion 422 formed in the rotor disk 420 and fitted with the root grooves 421 of the rotor disk 420.
As shown in FIG. 24 and FIG. 25, the stopper block 410 positioned at the cutout portion 422 is fixed by inserting a key 413 into holes 412 which are formed by key grooves 412a and 412b formed in a root portion 410a of the stopper block 410 and root portions 411a of the adjacent moving blades 411 in parallel to the turbine rotor axial direction. Thus, a centrifugal force applied to the stopper block 410 is supported by the adjacent moving blades 411 via the keys 413 to prevent the stopper block 410 from coming out.
When the stopper block 410 is provided in the turbine moving blade cascade 400, a weight balance is generally adjusted by reducing the weight of the moving blade which is arranged at a position symmetrical to the stopper block 410 with respect to the turbine rotor central axis.
The easiest method of adjusting the weight balance is to have a counter moving blade (moving blade positioned symmetrical about a point to the stopper block 410 with respect to the turbine rotor central axis) formed to have the same shape as the stopper block 410. But, the adoption of the above structure is not preferable because the steam passage portion is lost at two points on the circumference, and the performance decreases. Therefore, the weight balance of the conventional turbine moving blade cascade 400 is adjusted by locally fabricating the moving blades (e.g., Nos. 59 to 88 in FIG. 22) positioned on the side symmetrical to the stopper block 410 with respect to the turbine rotor central axis, namely, by forming the groove 415 to adjust the weight as shown in FIG. 26. The moving blades of which weights are adjusted by forming the groove 415 are called the weight-reduced moving blades hereinafter.
A conventional turbine moving blade cascade provided with a notch blade as a tightening part is described below.
FIG. 27 is a schematic view of a conventional turbine moving blade cascade 401 having a notch blade 440 as a tightening part as viewed from the upstream side in a turbine rotor axial direction. The fixing method of the notch blade 440 is basically same to the previously described fixing method of the stopper block 410, but when the notch blade 440 is used, pin holes are formed in the root portion of the notch blade 440 and the rotor disk, and locking pins are inserted into the pin holes so that it is configured to completely prevent the notch blade 440 from being floated up by a centrifugal force.
As described above, there is a tendency to adopt the notch blade as the tightening part to prevent a stage loss from increasing. Here, when design and manufacture are performed considering from the beginning a structure that, for example, 148 moving blades 411 (including the notch blade 440) are provided on the whole circumference, the weight balance can be adjusted easily. But, for example, when the structure having the stopper block as the tightening part is made to have a structure adopting the notch blade as the tightening part by an afterward design change or structure change, it cannot be performed easily because the weight balance must be adjusted considering the original state of the weight balance.
For example, in a case that a newly manufactured notch blade 440 is formed of the same iron and steel type material as the moving blades 411, countermeasures are considered after an unbalanced amount is reduced by fully replacing the weight-reduced moving blades used when the stopper block 410 is provided as the above-described tightening part by the regular moving blades 411. As one measure to reduce the unbalanced amount due to the provision of the notch blade 440, the notch blade 440 is formed of titanium, and some moving blades (e.g., Nos. 70 to 78 in FIG. 27) positioned on a side (hereinafter called the counter side) symmetrical to the notch blade 440 about a point with respect to the turbine rotor central axis are determined to be weight-reduced moving blades to adjust the weight balance.
As described above, when the stopper block or the notch blade is adopted as the tightening part in the conventional turbine moving blade cascade, plural weight-reduced moving blades are arranged on the counter side to adjust the weight balance. The weight-reduced moving blade is configured to have the groove in the moving blade as described above, but the groove cannot be formed to have a large size because of strength constraint. Therefore, the amount of the weight reduction is small even when the regular moving blade is replaced by the weight-reduced moving blade. Thus, it is necessary to arrange a large number of weight-reduced moving blades on the counter side.
When the design conditions for the moving blades are strictly restricted in view of strength, use of the weight-reduced moving blades might not be allowed. In such a case, it is necessary to adopt the stopper block as the tightening part or to adopt as the counter moving blade the moving blade having the same shape as the stopper block, and the design becomes to increase the stage loss.